Power fiber laser with mode conversion

ABSTRACT

A pumped-fiber laser comprising a multimode doped fiber ( 1 ) and a holographic spatial mode conversion device ( 3 ).

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The invention relates to power laser sources based on a pumpeddoped fiber whose core is multimode. Usually the fiber receives a beamemitted by a monomode oscillator. The continuous or pulsed beam outputby the fiber is therefore an amplified beam, but its power remainslimited. The invention is designed to produce compact and efficientfiber sources delivering a high-power beam with good beam quality.

[0003] 2. Background Art

[0004] The production of laser sources delivering power greater than 1kW has been demonstrated at the present time in several laboratoriesusing diode-pumped Nd:YAG rods. Two types of architecture result in suchperformance: single oscillator and “MOPA” configuration, consisting ofan oscillator, an amplifier and possibly a phase conjugation mirror. Theobject of the invention is to propose a diode-pumped fiber sourcearchitecture of the MOPA type. Continuous power greater than 100 W canbe obtained using quasi-monomode fibers. Taking these results intoaccount, we propose to produce sources delivering more than 1 kW ofpower using multimode fibers, the volume of the gain medium of which isvery much greater than that of a monomode fiber.

BRIEF SUMMARY OF THE INVENTION

[0005] The invention therefore relates to a pumped fiber lasercomprising a monomode laser oscillator transmitting a monomode laserbeam to a first end of a doped fiber, characterized in that the dopedfiber is multimode and in that it also includes a spatial modeconversion device receiving the beam. To guarantee diffraction-limitedbeam quality, a mode conversion device is introduced, which consists ofa nonlinear medium serving to record a dynamic hologram or possibly afixed (volume) hologram.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006] The various objects and features of the invention will becomemore clearly apparent from the following description given by way ofexample and from the figures which show:

[0007]FIGS. 1a to 1 c shows three embodiments of the invention;

[0008]FIG. 2 shows an embodiment of the invention with a holographicmedium used for mode conversion;

[0009]FIG. 3 shows an alternative version of the device shown in FIG. 2;

[0010]FIGS. 4a and 4 b show embodiments in which the multimode fibercomprises a multitude of doped cores; and

[0011]FIG. 5 shows an alternative embodiment of the invention comprisinga combination of multimode fiber and a spatial mode conversion device.

DETAILED DESCRIPTIOIN OF THE INVENTION

[0012]FIG. 1a shows a simplified example embodiment of the invention. Inthis example embodiment, the fiber is self-pumped. It comprises amultimode optical fiber whose core is doped so as to have an activemedium. The diameter of this core is preferably greater than 20 um oreven greater than 50 um. It may even be envisaged to have fibers whosecore diameter is greater than 80 Am. The optical fiber receives a laserbeam coming from an oscillator 2. This laser beam is monomode. The laserbeam travels along the optical fiber and is reflected by the phaseconjugation mirror 5, which reflects a beam self-pumping the fiber 1. Alaser beam is emitted by the fiber toward the semitransparent mirror 4or a polarization splitter which reflects the light energy received fromthe fiber 1 onto a mode conversion device 3.

[0013] This mode conversion device is produced in a nonlinear materialin which a volume hologram has been recorded and which will be explainedlater.

[0014] Thus, since the fiber 1 is multimode and very large in diametercompared with the fibers normally used in this type of laser, the beamemitted toward the device 3 is multimode—more specifically this beam isthe superposition of a multitude of plane waves of random amplitude andrandom phase. Under these conditions, using a fiber which is thusmultimode, a relatively powerful beam is emitted and, with the aid ofthe mode converter 3, an output beam OB is emitted which is of opticalquality such that the beam OB is monomode.

[0015]FIG. 1b is an alternative embodiment of the device shown in FIG.1, in which the phase conjugation mirror 5 has been replaced with afiber 5 a of the Brillouin fiber type. This Brillouin fiber is of thetype described in French patent No. 2 669 441 filed on Nov. 16, 1990.

[0016]FIG. 1c is another alternative embodiment of the device shown inFIG. 1a, in which the multimode fiber 1 is a Brillouin fiber.

[0017] The device shown schematically in FIG. 2 incorporates thefollowing elements:

[0018] a monomode laser oscillator 2;

[0019] a fiber amplifier 1;

[0020] a holographic medium 3 for mode conversion.

[0021] According to the embodiment in FIG. 2, the first end 1.1 of thefiber 1 receives the monomode beam emitted by the laser oscillator 2.The second end 1.2 of the fiber allows this beam, after being amplifiedin the fiber, to be transmitted toward the holographic medium 3. Thelatter also receives a portion of the monomode beam delivered by thelaser oscillator 2. The two beams interfere in the holographic mediumand, as will be explained below, this results in the energy of theamplified beam output by the fiber being transferred into the monomodebeam delivered by the laser oscillator 2.

[0022] The oscillator 2 consists of a monomode low-power (>1 W) source:the source is monofrequency, with a certain coherence length and withdiffraction-limited spatial beam quality. By way of example, this is adiode-pumped conventional oscillator or a fiber laser.

[0023] The multimode amplifier is therefore a fiber 1, the pumping ofwhich is provided by power diodes 8.1, 8.2, . . . 8.3, in a longitudinalor transverse configuration. The core of the fiber has a diameter, forexample, of 100 μm. According to the conventional technique, the fibercore, which constitutes the gain medium, is pumped by total reflectionof the pump wave at the cladding interface inside the fiber.

[0024] The mode conversion device 3 consists of a variable-indexnonlinear medium. The low-power beam B1 output by the oscillator and themultimode beam B2 output by the fiber amplifier interfere in the volumeof this medium. The two beams B1 and B2 are transmitted to the modeconversion device 3 via a monomode fiber 6. It is known that thetwo-wave type of interaction in the conversion device allows all theenergy of the intense multimode beam B2 to be transferred into themonomode beam B 1 provided that the two-wave gain coefficient of thematerial is high enough. This interaction with the beam clean-upfunction has been demonstrated with the following nonlinear mechanisms:photorefractive crystals of LiNbO₃, BaTiO₃, SBN, etc.; thermalnonlinearities: dyes, liquid crystals, etc.; Brillouin effect inmultimode fibers.

[0025] The device in FIG. 2 allows the intense beam B2 to be completelydepleted to the benefit of amplification of the monomode beam B1. If thehologram is a dynamic hologram, the material adapts to the slowvariations in the pattern of interference between the beam delivered bythe fiber 1 and the beam delivered by the laser oscillator 2.

[0026] The mode conversion device may also be made from a fixedholographic component, for example a photorefractive crystal followed bya procedure of fixing the photoinduced grating, or by a hologramrecorded in the volume of a photopolymer material. In this case, sincethe component is fixed, it no longer adapts to the change in theinterference pattern. The mode conversion is effective only if therelative phase between the various modes radiated by the fiber remainsfixed. This configuration does not require a reference beam forread-out. The system therefore is as shown in FIG. 3. The end 1.2 of thefiber 1 is coupled via the lens 7 to the mode conversion device 3. Thecombination of the fiber end 1.2, the Fourier lens and the holographicmode conversion component then constitutes a compact structure.

[0027] As an example, a source having a continuous power of greater than1 kW emitting at X=1.053 μm may be achieved under the followingconditions:

[0028] 1 W Yb-fiber oscillator;

[0029] 30 dB gain amplifier;

[0030] mode conversion by Rh-doped or LiNbO₃-doped BaTiO₃ crystalsensitized to λ=1.053 μm;

[0031] pumping power: 2 to 2.5 kW.

[0032] The reference beam delivered by the laser oscillator 2 istransmitted via a monomode fiber 6, the length of which is equal to thatof the amplifying fiber 1. This condition relaxes the constraint on thecoherence length of the oscillator.

[0033] The device of the invention allows a high power to be extractedfrom a multimode fiber amplifier. Under these conditions, all theassociated nonlinear effects (Brillouin, Raman, etc.) are reduced sincethe emitted power density remains less than the damaged threshold of theinterface.

[0034] A mode conversion device ensures coherent transfer of the energyemitted in a spatial mode with diffraction-limited quality.

[0035] The device proposed also carries out mode conversion if theamplifying fiber consists of an ordered or disordered assembly ofmonomode fiber cores, as shown in FIGS. 4a and 4 b.

[0036] Injection by the oscillator into an ordered array of monomodefiber cores takes place via a multiple-wave grating.

[0037] As shown in FIG. 5, the combination of multimode fiber and aspatial mode conversion device may constitute an oscillator emitting amonomode beam. In this case, the fiber 1 pumped by one or more diodes 8and the mode converter are placed in an optical cavity 9, 10.

[0038] It should be noted that the fiber may be apolarization-maintaining fiber. Otherwise, the mode converter must beable to handle both polarization components.

[0039] Finally, the laser may operate in continuous mode or pulsed mode.

1. A pumped fiber laser, comprising: a multimode doped fiber (1) havinga first end and a second end; a holographic spatial mode conversionmeans (3) configured to receive light from the multimode doped fiber; amonomode laser oscillator (2) configured to transmit a monomode laserbeam to the first end of said multimode doped fiber; and an opticalsplitter (4) placed between the monomode laser oscillator (2) and thefirst end of the multimode doped fiber (1), and configured to transmitpart of the monomode laser beam toward the multimode doped fiber and areflected part of the monomode laser beam toward the holographic modeconversion device (3) so that the energy is transferred from thereflected monomode laser beam to the part of the monomode laser beamcoming from the splitter such that the mode conversion device transmitsa monomode amplified beam.
 2. The laser as claimed in claim 1, whereinthe multimode doped fiber comprises: a core with a diameter larger than30 micrometers.
 3. (Cancelled)
 4. The laser as claimed in claim 1,further comprising: a phase conjugation reflection device (5) coupled tothe second end of the multimode doped fiber (1) and configured toreflect said monomode laser beam.
 5. The laser as claimed in claim 1,further comprising: at least one pumping light source configured totransmit a corresponding at least one pumping beam to the multimodedoped fiber.
 6. (Cancelled)
 7. The laser as claimed in claim 5, whereinthe holographic mode conversion device (3) comprises: a prerecorded modeconversion device configured to convert the reflected laser beam into amonomode beam.
 8. The laser as claimed in claim 1, wherein saidmultimode doped fiber comprises: a plurality of doped cores. 9-11.(Cancelled)
 12. The laser of claim 1, wherein said holographic spatialmode conversion means comprises one of: a variable index nonlinearmedium including a recorded volume hologram; and a fixed holographiccomponent, including one of a photorefractive crystal with a fixedphotoinduced grating and a hologram recorded in a volume of aphotopolymer material.
 13. The laser of claim 1, further comprising: anoptical cavity configured to contain said pumped-fiber laser such thatsaid holographic spatial mode conversion means is arranged in serieswith said optical cavity.
 14. A laser device, comprising: an opticalcavity; a pumped-fiber laser contained within said optical cavity; and aspatial mode conversion means in series with said optical cavity, saidspatial mode conversion means configured to convert a multimode beaminto a monomode beam.
 15. The laser device of claim 14, saidpumped-fiber laser comprising: a multimode fiber having a core diametergreater than 30 micrometers.
 16. The laser device of claim 14, whereinsaid spatial mode conversion means comprises one of: a variable indexnonlinear medium including a recorded volume hologram; and a fixedholographic component, including one of a photorefractive crystal with afixed photoinduced grating and a hologram recorded in a volume of aphotopolymer material.